Tunneling Lifetime of the ttc/VIp Conformer of Glycine in Low-Temperature Matrices

Conformer ttc/VIp of glycine and glycine-N,N,O-d(3) has been prepared in low-temperature Ar, Kr, Xe, and N-2 matrices by near-infrared (NIR) laser irradiation of the first OH stretching overtone of conformer ttt/Ip. Glycine (and glycine-N,N,O-d(3)) ttc/VIp was found to convert back to ttt/Ip in the dark by hydrogen-atom tunneling. The observed half-lives of ttc/VIp in Ar, Kr, and Xe matrices at 12 K were 4.4 +/- 1 s (50.0 +/- 1 h), 4.0 +/- 1 s (48.0 +/- 1 h), and 2.8 +/- 1 s (99.3 +/- 2 h), respectively. In correspondence with the observation for the cis-to-trans conversion of formic and acetic acid, the tunneling half-life of glycine ttc/VIp in a N-2 matrix is more than 3 orders of magnitude longer (6.69 X 10(3) and 1.33 X 10(4) s for two different sites) than in noble gas matrices due to complex formation with the host molecules. The present results are important to understand the lack of experimental observation of some computationally predicted conformers of glycine and other amino acids

HeI PHOTOELECTRON SPECTROSCOPY AND ELECTRONIC STRUCTURE OF ALKYL-LITHIUM CLUSTERS

Author Institution: Department of General and Inorganic Chemistry, E\""{o}tv\""{o}s UniversityHeI photoelectron spectra of some alkyl-lithium tetrameric and hexameric clusters have been recorded. In the low energy region (ca. 6.0-9.0 eV) bands have been assigned to ionization from the Li-C cluster orbitals. Ionization from the triply and doubly degenerate orbitals of the tetramers ($T_{d}$ symmetry) and hexamers ($D_{3d}$ symmetry), respectively, causes Jahn-Teller (JT) distortion of the clusters. Ab initio quantum chemical calculations have been performed to understand the nature and extent of the JT distortions

An automated framework for NMR chemical shift calculations of small organic molecules

When using nuclear magnetic resonance (NMR) to assist in chemical identification in complex samples, researchers commonly rely on databases for chemical shift spectra. However, authentic standards are typically depended upon to build libraries experimentally. Considering complex biological samples, such as blood and soil, the entirety of NMR spectra required for all possible compounds would be infeasible to ascertain due to limitations of available standards and experimental processing time. As an alternative, we introduce the in silico Chemical Library Engine (ISiCLE) NMR chemical shift module to accurately and automatically calculate NMR chemical shifts of small organic molecules through use of quantum chemical calculations. ISiCLE performs density functional theory (DFT)-based calculations for predicting chemical properties—specifically NMR chemical shifts in this manuscript—via the open source, high-performance computational chemistry software, NWChem. ISiCLE calculates the NMR chemical shifts of sets of molecules using any available combination of DFT method, solvent, and NMR-active nuclei, using both user-selected reference compounds and/or linear regression methods. Calculated NMR chemical shifts are provided to the user for each molecule, along with comparisons with respect to a number of metrics commonly used in the literature. Here, we demonstrate ISiCLE using a set of 312 molecules, ranging in size up to 90 carbon atoms. For each, calculation of NMR chemical shifts have been performed with 8 different levels of DFT theory, and with solvation effects using the implicit solvent Conductor-like Screening Model. The DFT method dependence of the calculated chemical shifts have been systematically investigated through benchmarking and subsequently compared to experimental data available in the literature. Furthermore, ISiCLE has been applied to a set of 80 methylcyclohexane conformers, combined via Boltzmann weighting and compared to experimental values. We demonstrate that our protocol shows promise in the automation of chemical shift calculations and, ultimately, the expansion of chemical shift libraries